Heater with boron nitride coating



Sept. 29, 1970 F. J. SCHIAVONE HEATER WITH BORON NITRIDE COATING FiledJune 24, 1968 I6 I I 22 F/G 3 I INVE/VT'OI? FRANK .1. SCH/A VO/VE UnitedStates Patent ()1 iice 3,531,678 Patented Sept. 29, 1970 US. Cl. 313-6373 Claims ABSTRACT OF THE DISCLOSURE A heater for devices such aselectron discharge devices comprising a wire or similar substrate havingthereon an electrically insulating thermally conductive coating of boronnitride.

BACKGROUND OF THE INVENTION This invention relates to heater elementssuch as filament wires of electron discharge devices or the like, andhas particular reference to heater wires having deposited thereon acoating of boron nitride, preferably isotropic in nature, for providingthe heater wire with improved mechanical and electrical properties.

In the manufacture of heaters such as filament wires of electron tubesand the like, it has been necessary to coat the filament with a suitableinsulating material which will protect the filament while preventing theoccurrence of electrical shorts under adverse conditions of use.Filaments of tungsten are commonly used and have been provided withcoatings of alumina formed by sintering alumina powder to the surface ofthe tungsten wire at high temperatures.

However, serious problems are encountered when using alumina as theprotective coating since there exists a considerable difference inthermal expansion between tungsten and alumina. Alumina has an expansioncoefficient of 8.512.5 l C., while that of tungsten is 4.4 l0 C.Therefore, separation often occurs during use of a heater embodying analumina-coated tungsten wire.

It is essential, of course, that the coating be highly transmissive tothe heat generated by the filament so that, in an electron tube, forexample, the filament may be utilized efficiently to heat an adjacentelectron emitter to the point of electron emission.

SUMMARY OF THE INVENTION In accordance with the present invention, thereis provided a novel heater structure which comprises a tungsten filamenthaving thereon a coating of boron nitrile, preferably isotropic innature, which coating effectively insulates the filament wire,elficiently conducts heat from the filament wire to the ambient, and hasa thermal expansion coefiicient which closely matches that of theunderlying tungsten.

BRIEF DESCRIPTION OF THE DRAWING FIG. 1 is an elevational view of acoated heater embodying the invention, showing the coating partiallybroken away;

FIG. 2 is a sectional view taken on line 22 of FIG. 1; and

FIG. 3 is an axial sectional view of a cathode structure embodying theinvention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring more particularly tothe drawing, wherein like characters of reference designate like partsthroughout the several views, there is shown in FIGS. 1 and 2 a heaterelement such as a tungsten wire 12 having thereon a coating 14 of boronnitride. The heater may be of any other selected configuration, rod,sphere, disc, or the like, and is suitably connected to a source 16 offilament power SP whereby it may be heated to temperatures as high as1500 C., for example.

One example of actual use of a heater element of a type embodying thisinvention is shown in FIG. 3 wherein a coated filament wire 18 coiled ina predetermined configuration is disposed within a hollow cylindricalmetal cathode 20 which is provided on its outer surface with a coating22 of electron emitting material. The cathode 20 may be nickel, and theelectron emitting material may be barium-strontium oxide, for example.The ends of the filament wire 18 are mounted upon respective leads 24and 26 whereby connection may be made to a suitable source of filamentpower. Thus, when the filament wire is heated by flow of currenttherethrough, the generated heat is transmitted through the boronnitride coating into the ambient, whereupon the cathode 2 0 will becomeheated to a temperature level which will cause electrons to be emittedfrom coating 22. A cathode structure such as illustrated in FIG. 3 is acommonly used component in many types of electron discharge devices.

The boron nitride coating 14 is an excellent insulator and preventsshorting which may occur, for example, when vibration, mechanical shock,or other influence causes the heater wire 18 to move into physicalcontact with the cathode cylinder 20. Boron nitride also is an excellentconductor of heat and, in fact, is better in this respect than alumina.Thus, more eflicient transmission of heat to the cathode cylinder isachieved.

Furthermore, tungsten and boron nitride have very similar coefiicientsof expansion, being 4.4X10- C. for tungsten and 4.0 10 C. for boronnitride. This is a decided improvement over alumina which has acoefficient of expansion of 8.5l2.5 l0- C. Thus, under severe conditionsof use, continued expansion and contraction of the tungsten and boronnitride will not produce undesirable separation as often occurs whenalumina is used as the insulating coating.

The coating may be relatively thin because of the extreme density of theboron nitride material, a satisfac tory thickness being in the range ofabout .0O1.006 inch, for example.

The boron nitride layer or coating 14 may be deposited upon the heaterelement 12 by any of the known methods which result in the production ofanisotropic boron nitride, such as, for example, the method disclosed inUS. Pat. No. 2,824,787 wherein, briefly, a borate ester such as methylor ethyl borate and ammonia are reacted at a temperature between 850 C.and 900 C. and at atmospheric pressure to produce a white, finelydispersed powder which is thereafter treated with ammonia at atemperature between 800 C. and 1300 C. to produce powdered boronnitride. Such a powder may be applied under heat and pressure to aheater element to form a coating thereon.

Another known method, as taught by Basche in US. Pat. No. 3,152,006,results in the production of a solid boron nitride coating on asubstrate by vapor-phase reaction between ammonia and boron trichlorideat a temperature between about 1450 C. and 2300 C. at a pressure belowabout 50 mm. of mercury.

Such coatings are anisotropic, having been found to possess excellentthermal conductivity, electrical resistance, and other characteristicswhich, however, are better in one direction than in another due to theanisotropicity of the material. Therefore, to obtain optimum resultswhen applying an anisotropic boron nitride coating on a heater wire,best results are obtained when the coating is so applied that thedirection of maximum thermal conductivity is perpendicular to thesurface of the coating.

Even better results are obtained, however, by applying an isotropicboron nitride coating to the heater element. This may be done by amethod disclosed in pending U.S. patent application Ser. No. 582,686 nowabandoned and assigned to the same assignee as the present invention,which teaches the production of isotropic boron nitride by the reactionof ammonia gas with an organic boron compound such as trimethyl borate,dimethyl boric acid, triethyl borate, trimethyl triborine trioxane ortetramethoxyborine, or others, in a vapor deposition furnace at anelevated temperature in the range of about 1200.2300= C. at a pressureof less than about 100 torr, whereby when the gaseous reactant agentsare co-mingled and then directed upon a substrate, such as heaterelement 12, for a selected length of time, they will form on the heaterelement a solid isotropic boron nitride coating of a thickness primarilydependent upon the time cycle.

The electrical resistivity of the isotropic boron nitride coating 14ranges from about 10 ohm/ cm. at room temperature to about 10 ohm/cm. at1350 0, making it one of the best high temperature electricallyinsulating materials known. Furthermore, because of its isotropiccharacteristics it can be easily, efficiently and permanently sealed tothe metal heater wire, and can be readily made in various sizes andshapes and can be of practically any configuration, regardless of theradius of curvature employed.

The heat during the process of depositing the isotropic boron nitridelayer can be generated by an external source or may be generated withinthe heater element by impressing a suitable voltage to the elementduring the deposition process.

Additional details of the methods of applying the boron nitride coating14 to the heater element 12, and of the characteristics of the coatingmay be obtained from the above-mentioned patents and application. Themethods do not constitute a part of this invention.

It is to be understood that various modifications and changes in theinvention disclosed herein may be made by those skilled in the artwithout departing from the spirit of the invention as expressed in theappended claims.

I claim:

1. A heater element comprising a tungsten substrate having thereon acoating of isotropic boron nitride, the direction of maximum thermalconductivity of the coating being perpendicular to the surface thereof.

2. A cathode structure comprising an electron emissive member, and aheater element adjacent said member for heating the member to atemperature level at which electrons are emitted therefrom, said heaterelement comprising a tungsten substrate having thereon a coating ofisotropic boron nitride, the direction of maximum thermal conductivityof the coating being perpendicular to the surface thereof.

3. In an electron discharge device, a cathode structure comprising ahollow metal cathode cylinder having a layer of electron emissivematerial on the outer surface thereof, a filament wire within thecathode cylinder for heating the cathode cylinder and emissive materialto a temperature at which electrons are emitted by the material, aninsulating heat transmitting coating of isotropic boron nitride on saidfilament wire, the direction of maximum thermal conductivity of thecoating being perpendicular to the surface thereof, and means forconnecting said filament wire to a source of power external to thedevice.

References Cited UNITED STATES PATENTS 3,119,897 1/1964 Coper 313337 X3,152,006 10/1964 Basche 117-106 3,178,308 4/1965 Oxley et al. 117l063,259,783 7/1966 Norris et a1. 313--337 3,273,005 9/1966 Lafierty313-337 X 3,321,337 5/1967 Patterson ll7l06 X JAMES D. KALLAM, PrimaryExaminer A. J. JAMES, Assistant Examiner U.S. Cl. X.R.

